1. Field of the Invention
This invention generally relates to systems and methods for inspection of a specimen such as a patterned wafer. Certain embodiments relate to inspection systems configured to separately detect different portions of light scattered from a specimen such that angular information about the different portions of the light is preserved.
2. Description of the Related Art
Many different types of inspection tools have been developed for the inspection of semiconductor wafers. The inspection tools may be categorized generally according to the types of specimen that they are designed to inspect. For example, one category of inspection tools is generally designed to inspect unpatterned semiconductor wafers. Since these tools are optimized for inspecting unpatterned wafers, these tools are generally not capable of inspecting patterned wafers for a number of reasons. For example, many unpatterned wafer inspection tools are configured such that all of the light collected by a lens or another collector is directed to a single detector that generates a single output signal representative of all of the light collected by the lens. Therefore, light scattered from patterns or other features on the specimen will be combined with other scattered light. As such, the single detector may become saturated and, consequently, will not yield signals that can be analyzed for defect detection. In addition, even if the single detector does not become saturated, the light scattered from patterns or other features on the wafer can not be separated from other scattered light thereby hindering, if not preventing, defect detection based on the other scattered light.
Patterned wafer inspection is of particular interest and importance to the semiconductor industry because processed semiconductor wafers usually have a pattern of features formed thereon. For example, fabricating semiconductor devices such as logic and memory devices includes processing a semiconductor wafer using a number of semiconductor fabrication processes to form various features and multiple levels of the semiconductor devices. One example of a semiconductor fabrication process is lithography, which typically involves transferring a pattern to a resist arranged on a semiconductor wafer. Additional examples of semiconductor fabrication processes include chemical-mechanical polishing, etch, deposition, and ion implantation. Although inspection of unpatterned wafers, or “monitor wafers,” which have been run through a process tool, may be used as a gauge for the number and types of defects that may be found on patterned wafers, or “product wafers,” defects detected on monitor wafers do not always accurately reflect the defects that are detected on patterned wafers after the same process in the process tool. Inspection of patterned wafers after such processing is, therefore, important to accurately detect defects that may have been formed on the wafer during, or as a result of, processing.
The results of such inspection may be used to monitor and control semiconductor fabrication processes. Therefore, inspecting patterned wafers or product wafers may provide more accurate monitoring and control of processes and process tools than inspection of monitor wafers. Successful fabrication of semiconductor devices is often limited by the presence of defects in the semiconductor devices. If the fabrication processes can be monitored and controlled for defects, the yield of such processes may be maintained or improved. Furthermore, monitoring semiconductor fabrication processes over time has become increasingly important in the industry to improve or maintain yield as the dimensions of semiconductor devices shrink.
Many inspection tools have been developed for patterned wafer inspection. In many cases, the optical design of such tools can be substantially more complex than that of unpatterned wafer inspection tools. For example, one patterned wafer inspection tool utilizes spatial filters to separate light scattered from patterned features from other scattered light such that the other scattered light may be separately detected. Since the light scattered from patterned features depends on various characteristics of the patterned features such as lateral dimension and period, the design of the spatial filter also depends on such characteristics of the patterned features. As a result, the spatial filter must be designed based on known or determined characteristics of the patterned features and must vary as different patterned features are being inspected. Consequently, although such an inspection tool may provide patterned wafer inspection capability, there are some drawbacks due to the complex optical design.
Accordingly, it may be advantageous to develop a patterned wafer inspection system that has a relatively simple optical design. In addition, it may be advantageous to develop a patterned wafer inspection system that also has unpatterned wafer inspection capability thereby increasing the flexibility of the inspection system, which may become increasingly important as the spatial limitations on inspection tools become more stringent based on clean room costs and for integration of inspection modules into process tools.